1. Field of the Invention
The present invention relates to an anti-lock control apparatus and, more particularly, to an improvement in an anti-lock control apparatus of the so-called two-channel diagonal simultaneous control type which is suitable for use in, particularly, four-wheel vehicles and in which the front and rear brakes on the right and left sides are diagonally joined so that the front brake and the rear brake which are located on each diagonal line may be simultaneously controlled.
2. Description of the Related Art
It has been proposed to provide various types of anti-lock control apparatus for use in a brake system in which a brake line serving to brake a left front wheel and a right rear wheel is independent of a brake line serving to brake a right front wheel and a left rear wheel. For example, a known type of anti-lock control apparatus is arranged so that, if the front or rear wheel in either brake line shows a tendency to wheel-lock, then pressure decreasing control may be exerted over the fluid pressure of a front brake and the fluid pressure of its corresponding rear brake at the same time.
However, such a conventional type of anti-lock control apparatus leaves the following problem to be solved. When a vehicle is being braked while it is running on a so called split road surface, whose right and left sides exhibit different friction coefficients .mu., or when an anomalous distribution of the braking force occurs between the front and rear wheels, a rear wheel may show a tendency to wheel-lock prior to the corresponding front wheel. In this case, since pressure decreasing control is provided over the fluid pressure of the front wheel and that of the rear wheel at the same time, the fluid pressure of the front wheel, which has not yet shown the tendency to wheel-lock, would be decreased to an excessive extent, and the resultant braking force might fall short of the total braking force required to stop the vehicle.
Incidentally, the related arts are disclosed in U.S. Pat. No. 4,752,104 and U.S. Pat. application Ser. No. 07/213,127 (patent pending).